Lactones and cyclic esters are obtained by the thermal depolymerization of the corresponding linear polyesters accompanied by ring closure. For such processes, the polyester is heated at an elevated temperature in the presence of a catalyst. The reaction is carried out under reduced pressure and the macrocyclic compound and other volatile products formed during the course of the depolymerization are removed from the reaction zone as they are formed. Due to the poor heat transfer within the highly viscous reaction mass, until recently it was only possible to conduct the reaction as a batch-type operation--thus severely limiting the commercial utility of the process. With the discovery of the process of U.S. Pat. No. 4,165,321, however, the continuous and semi-continuous production of macrocyclic compounds by thermolysis of polyester is now possible.
Chlorides, nitrates, carbonates and oxides of magnesium, manganese, iron, cobalt and tin (all in the divalent state) are disclosed to be effective catalysts for the batch depolymerization of linear polyesters in U.S. Pat. No. 2,092,031. For the process of U.S. Pat. No. 4,165,321 Lewis metal salts such as the oxides, hydroxides, halides, or carboxylates, of Group IIIa, IVa, IVb, Va, VIIb and VIII metals are indicated to be useful catalyst. Yasakawa et al. reported the use of lead catalysts (oxide, hydroxide, carbonate, nitrate, borate or organic acid salts) for the preparation of large ring lactones via thermal depolymerization in Chemical Abstracts, Vol. 78 (1973), 158966q and 158968s. Cyclic esters are also obtained via thermal degradation of polyesters using SnCl.sub.2.2H.sub.2 O (Chemical Abstracts Vol. 86 (1977), 156163s) and in the U.S. Pat. Nos. 4,105,672, 4,136,098 and 4,157,330 a tin carboxylate or an organotin compound in conjunction with an O,O-dialkyl-(3,5-di-t-butyl-4-hydroxybenzyl)phosphonate are employed to catalyze the reaction. In somewhat related procedures, cyclic ester anhydrides of .alpha.-hydroxycarboxylic acids are formed in vacuo by depolymerizing the corresponding linear polymer at 200.degree.-240.degree. C. in the presence of lead (II) stearate (British Pat. No. 1,108,720).
While all of the aforementioned metal compounds catalyze the depolymerization and ring closure to varying extents, whether the process is conducted as a batch or continuous operation, they are not without certain disadvantages. In the first place, many of these catalysts are insoluble or have limited solubility in the reaction medium and give poor yields of the desired macrocyclic products. Even when acceptable yields are obtained the rates of reaction are often slower than desirable so that process equipment can be utilized at only a fraction of its capacity. Efforts to increase the reaction rate by raising the temperature of reaction are only partially successful since this often leads to destructive thermal decomposition and/or excessive foaming, particularly in batch-type operations.
Additionally, it is virtually impossible to completely eliminate the presence of some heavy metal contaminants in the macrocyclic product. The presence of even trace amounts of heavy metal residues can impart undesirable discoloration to the product and, if the product is stored, may promote degradation of the macrocyclic compound or other components formulated therewith. An even more serious problem exists when the catalyst is derived from a toxic metal, such as lead. Toxic metal contaminants cannot be tolerated in most applications where macrocyclic compounds are utilized and this either precludes the use of the products in these application areas or makes it necessary to subject the macrocyclic product to costly and time consuming post-treatment operation(s) capable of eliminating the metal residue.
Aluminum oxide has been used for depolymerizations carried out at atmospheric pressure using superheated steam (Czech Pat. No. 108,726) and the use of aluminum is reported in Japanese Patent Sho 36[1961]-1375 for the thermal depolymerization of polyesters to form cyclic esters and lactones. Aluminum alkoxides derived from simple alcohols are used for the preparation of large-ring lactones and large-ring ethylene dioates in Japan Pat. No. 72 25,071 and Japan Kokai Tokkyo Koho No. 79,115,390, respectively.